Image formation apparatuses using electrophotography, such as photocopiers, printers, fax machines, etc., are well known, as disclosed for example in Japanese Patent Application Publication Nos. H10-74023 (JP-H10-74023-A) and H11-52783(JP-H11-52783-A).
To form images, such conventional apparatuses first form an electrostatic latent image on a surface of a photosensitive drum of an image bearer. The latent image is then developed into a visible image using toner as a development agent. The thus-developed image is transferred onto a sheet of recording material and is fixed thereon by a fixing device using heat and pressure to complete the image formation process.
The fixing device includes a fusing rotation member composed of opposed rollers, belts, or a combination of rollers and belts, sandwiches the recording sheet therebetween, and fuses the toner image onto the recording sheet by applying heat and pressure. The fixing device includes a heater, power supplying to which is controlled, to generate heat. The fixing device calculates a duty of electricity (i.e., a power-turn-on time period) supplied to the heater to control the temperature of the fixing device members at each control cycle. The fixing device conducts zero-crossing control based on the duty thus calculated while also performing soft-start control based on phase control to avoid abnormalities such as flicker, etc.
However, supplying a heater with power using soft-start control while gradually increasing a phase angle means that, near duties of 0% and 100%, one of the power-turn-on duties necessarily becomes discontinuous. Consequently, target temperatures cannot be maintained at duties near 0% or 100%, resulting in defective fusing. In recent years, fixing device members having a low heat capacity are used to reduce a warm-up time to save energy. As a result, control is bifurcated for the same product, with a higher duty used to start up quickly while a lower duty used only to maintain heat due to upgrading of thermal efficiency. Such an arrangement aggravates the problem of inability to maintain target temperatures at duties of near 0% and 100% described above.